This invention relates generally to semiconductor devices, and more specifically, to non-volatile memory (NVM) devices.
Non-volatile memory (NVM) devices are desirable memories because they retain their charge even when disconnected from a power source. Computers and cellular telephones, for example, use NVM devices to preserve information when turned off. One type of NVM cell is a SONOS (silicon-oxide-nitride-oxide-silicon) device, which has a silicon, (top) oxide, nitride, and (bottom) oxide stack listed from top to bottom, formed over a semiconductor, such as silicon, substrate.
One method to program and erase a SONOS device is to use tunneling. Typically, the bottom oxide of SONOS is used to prevent electron movement between the substrate and the nitride. However, the bottom oxide is designed to be thin enough (e.g., approximately 18 to 30 Angstroms in thickness) so that electrons can be transported by tunneling through the bottom oxide when an electric field is generated when biasing the SONOS device. In order to quickly programming of the SONOS device, as required in many NVM applications, the bottom oxide should be less than or equal to approximately 22 Angstroms. However, in this thickness range the electrons can easily travel (leak) from the nitride layer into the substrate and therefore, the SONOS device can easily lose data. In addition, read disturb problems can arise.
If a thicker bottom oxide (e.g., approximately 50-100 Angstroms in thickness) is used, hot carrier injection (HCI) and hot hole erase (HHE) can be performed. An inversion layer is created in a channel region of the SONOS device, which transmits electrons from a source region to a drain region. Electrons are injected into the nitride through the bottom oxide. Holes, which are created in a drain depletion region of the device, are injected into the nitride to erase the SONOS device by HHE. Although this program and erase scheme is desirable because it is a fast process (e.g., on the order or microseconds), the location of injected electrons during programming and injected holes during erasing may differ, making it difficult to fully erase the SONOS device. HHE also damages the substrate to tunnel oxide interface and any overlying dielectrics. This damage can lead to reliability concerns n the SONOS device. Therefore, a need exists for a programming and erasing scheme that does not have the negative advantages of the above processes.